The closest thing I can think of 字统 is https://zi.tools. (There's also 字海 http://www.yedict.com which you cannot search Ideographic Description Character sequence and GlyphWiki which I trust much less.)
You can search an Ideographic Description Character sequence in 字统 and it will automatically directs to the corresponding character page. For example, searching ⿳立日心 will lead you to the page of 意. These would satisfy ad hoc use.
If the character exists but not encoded in Unicode, there's not single character and 字统 draws it with html-svg.
In 字海 you cannot search the IDC sequence but on each page of the character, the IDC sequence is also given.
I would like to combine multiple radicals into a single character, disregarding if they actually exist or not.
I might be wrong but is it technically impossible to output a single character if it doesn’t exist? Pairing with a specific font, those characters could be mapped to PUA. But this is arbitrary. The IDC sequence is also not standardized. For example, 字统 gives a different one for biang2 𰻞. By the way, this character is now in Unicode Ext-G.
A more realistic goal is to set eyes on only the existing characters that are Unicode-coded.
I don't have the data from 字统 but since they don't have a robots protocol, it's safe to scrawl their website. They have the IDC sequence for all characters, Unicoded or not.
They have one page for each character. Click on the big glyph will copy the single character or the IDC sequence, and the IDC sequence is also given as a string under the glyph. For example,
After you get a mapping from the IDC sequence to the character, you can easily build a rime input schema. That way you'll get a 输入法. Or write a small python script to use the mapping on command line.
If you want to save the trouble of scrawling the website, try https://github.com/hfhchan/ids, "A list of Ideographic Description Sequences (IDS), created manually or by exporting compositional data from glyphs made under the hkcs namespace on GlyphWiki." Another one is https://github.com/cjkvi/cjkvi-ids. The data are in txt files and look like
U+7E2E 縮 ⿰糸宿
U+8E5C 蹜 ⿰𧾷宿