Something like this (tested in opengl renderer)
        case SDL_BLENDMODE_ALPHA:
            data->glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
            data->glEnable(GL_BLEND);
            data->glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_ONE, GL_ZERO);
            break;

This allows applying alpha masks to texture using alpha-channel from other texture, like this:
                SDL_SetRenderTarget(gRenderer, texBuf);

                SDL_SetTextureBlendMode(baseTexture,SDL_BLENDMODE_NONE);
                SDL_SetTextureBlendMode(maskTexture,SDL_BLENDMODE_ALPHA);
                SDL_RenderCopy(gRenderer,baseTexture, NULL, NULL);
                SDL_RenderCopy(gRenderer,eggTexture, NULL, NULL);

After that we'll have baseTexture with alpha channel from maskTexture in texBuff.

Or maybe there's a better way to do this?